My apartment complex is saying they never received my 60 day notice! My moving date is in 3 weeks... HELP!?

Back in December, the complex left a note on my door saying that it was time to renew my lease or turn in a 60 day notice. I called the complex and told them that I work crazy hours and am not able to come to the office during office hours to get the form I need to sign to vacate my apartment. They told me they would leave the "intent to vacate" form on my door. When I got home that night I had the form, signed it, filled out the info requested...put it in the envelope with my rent for that month and dropped it off all in one envelope in the rent drop box. Now here we are three weeks before my move out date and they are leaving notices on my door saying they never received my notice. I called them today and told them the whole situation, and they told me that they would speak with the manager and get back to me. They haven't called me and and aren't answering when I call. What am I supposed to do?! It's their word versus mine! HELP!